Low sulfur recarburizer coke is a type of coke used in the production of high quality steels. Its purpose is to increase the carbon content of the steel without introducing any extraneous contaminants, especially sulfur and nitrogen. Historically, steel producers and recarburizer marketers have used crushed scrap graphite (graphitized premium coke) as the major source of recarburizer coke. However, this source has steadily declined as scrap rates in the graphite electrode production, and electric arc furnaces have been reduced. A market now exists for alternative sources of recarburizer coke with very low levels of contaminants.
It would be possible, of course, to manufacture high quality, premium coke, calcine and graphitize this material and use it as recarburizer coke. However, premium coke is too valuable in its use as electrodes for the manufacture of steel and, it would be uneconomic to use this material as recarburizer coke. Prior to graphitization, premium coke usually contains substantial amounts of sulfur and nitrogen, up to 0.3 to 0.5 or higher weight percent sulfur and nitrogen in similar quantities. Thus, ungraphitized premium coke would not be suitable for use as recarburizer coke even if economics would permit its use. Another type of coke which is manufactured in substantial quantities is so called aluminum grade coke, that is, coke which is used in manufacturing electrodes for use in the production of aluminum. This coke also contains substantial amounts of sulfur and nitrogen which make it unsuitable for use as recarburizer coke.
It has been found that pyrolysis tar can be processed to produce recarburizer coke. In order to use pyrolysis tar for this purpose, it first must be subjected to hydrogenation to reduce the sulfur and nitrogen content of the tar. Unfortunately, hydrotreating of pyrolysis tars can cause reactor bed plugging and a high rate of heat generation in the reactor, which makes it difficult to control the reactor temperatures.
In accordance with this invention, a mixture of pyrolysis tar and petroleum distillate is catalytically hydrogenated to reduce the sulfur and nitrogen content to low levels, the hydrotreated tar is then thermally cracked to provide a thermal tar which is subjected to delayed coking and the delayed coke is calcined to provide a recarburizer coke product containing not more than 0.1 weight percent sulfur and not more than 0.1 weight percent nitrogen. The process of the invention is effected without reactor bed plugging and without a high rate of heat generation in the reactor.